UC San Diego

This dissertation reports infrared (sub-THz through visible/UV) spectroscopy studies of two novel semiconductor systems: dilute (ferro)magnetic semiconductors (DMSs) and topological insulators (TIs). The first part addresses work on thin films of the canonical DMS, (Ga,Mn)As. In this system there is general consensus that the ferromagnetic mechanism is mediated by itinerant holes introduced by the Mn doping. However, the details of this exchange have been widely contested, and center on the character of the electronic states in the vicinity of the Fermi level. Through detailed infrared studies, we show the important role played by Mn impurity states in the transport dynamics and ferromagnetic interaction. In the second part, I focus on epitaxial films of the TI Bi₂Te₃, as well explore Mn or Sb doped Bi₂Te₃ films. In the former case, Mn doping leads to a ferromagnetic transition below Tc=15 K. Contrasting with the spectroscopic hallmarks of carrier mediated ferromagnetism in the canonical DMS (Ga,Mn)As, we show charge carriers do not play an important role in mediating ferromagnetism in this dilute magnetic topological semiconductor. While both pristine and Mn-doped Bi₂Te₃ are revealed to have significant concentrations of bulk charge carriers, we show that Sb doping in Bi₂Te₃ is highly effective in lowering the Fermi level with respect to the conduction band edge to reduce bulk carriers. Moreover, the Drude spectral weight in Sb doped Bi₂Te₃ is sufficiently small as to be consistent with the response of topological Dirac surface state (SS) charge carriers. This latter assertion is evidenced by establishing both the experimental bulk optical band gap, and theoretical sum rule constraints on the Drude oscillator strength of the SS response